1. Field of the Invention
The present invention relates to a carbon black master batch for use in a toner used in electrophotography and the like and a toner for use in electrophotography (electrostatic charge image development).
2. Description of the Related Art
A large number of methods are known as the electrophotography method as described in Patent Document 1; in general, taking advantage of photo conductive materials, electric latent images are formed on photoreceptors by means of various procedures, the electric latent images are developed by use of toner, the toner images are transferred electrostatically, according to need, to recording media such as paper, and copied matters are obtained by fixing the toner images by applying heat, pressure, or solvent vapor.
Additionally, a variety of methods have hitherto been proposed for the methods of developing by using toner and the methods of fixing toner images, and the methods appropriate to the respective image formation processes have been adopted.
Furthermore, in these years, for the above described electrophotography, advanced techniques including speed up of copying, enhancement of image quality, and high stability have been demanded.
On the other hand, the toner used in the above described electrophotography is obtained by mixing, melt-kneading, subsequent crushing and classifying of a raw material containing a binder resin, dyes/pigments as colorants, an electric charge control agent, wax and the like.
In this connection, an important point is that the additives including dyes/pigments and electric charge control agent are made to be dispersed in the binder resin in a homogeneous condition.
In other words, the poor dispersion of these additives degrades the charging characteristics, leads to the image characteristics variation, and makes a cause for generation of image deterioration including fogging and toner scattering.
As a typical production method of toner, for example, a method is adopted in which the raw materials including colorants, an offset preventing agent and an electric charge control agent are dispersed by melt-kneading in a binder resin such as styrene resin, styrene-(meth)acrylic resin, polyester resin or epoxy resin, then the kneaded material is pulverized by means of a pulverizing mill, the pulverized material is classified by means of a classifying apparatus, and thus a toner having a desired particle size is obtained.
In such a case, carbon black is widely used as a colorant for use in black toner.
However, when the carbon black is not fully dispersed in such a toner produced as described above, the carbon black tends to expose itself on the toner surface, which sometimes leads to the partial reduction of the electric resistance of the toner because carbon black is generally a conductor, thus varying the toner charge amount.
Consequently, the variation in the image characteristics, fogging and toner scattering tend to occur, resulting in poor recording stability.
Several measures have been proposed for such problems associated with the poor dispersion.
Examples of such measures include a method in which preliminarily prepared is a so-called master batch that is made to contain in a binder resin dyes/pigments in higher concentrations than the concentrations of dyes/pigments in the toner, and the above described master batch is used as a colorant.
A primary method for producing a master batch is the one in which a binder resin and dyes/pigments are mixed together preliminarily, and then further subjected to two-roller kneading.
However, the associated state of affairs is such that in this method the dyes/pigments are not fully dispersed in the binder resin and thus no sufficiently satisfactory stability is attained.
Additionally, a method similar to that described above, for example, a method described in Patent Document 2 is the one in which a master batch produced on the basis of a method similar to that described above is pulverized, then the resin remaining to be blended is blended together, and the mixture thus obtained is melt-kneaded; in this described method, too, the extent of dispersion of the dyes/pigments in the master batch is expected to govern eventually and largely the dispersion behavior of the dyes/pigments in the toner, accordingly leaving some doubt as to the provision of beneficial effects.
Additionally, Patent Document 3 describes a method in which a strong shearing force is applied in the course of the production of a master batch that is made to contain high concentrations of dyes/pigments in a binder resin, while on the contrary a weak shearing force is applied in a kneading process for dilution; however, it is probably impossible to obtain effects leading to attainment of a satisfactory dispersion.
Other relevant methods include, for example, another method for producing a master batch described in Patent Document 4 in which the raw materials are dissolved in a solvent to be mixed together, and then the solvent is evaporated to yield a master batch; no sufficient shearing force can be applied merely by stirring the solution mixture, and hence some doubts are raised as to whether these other methods can attain a satisfactory dispersion of such pigments as carbon black for use in a toner or not.
[Patent Document 1]
U.S. Pat. No. 2,297,691 specification.
[Patent Document 2]
Japanese Patent Laid-Open No. 62-30259.
[Patent Document 3]
Japanese Patent Laid-Open No. 63-205664.
[Patent Document 4]
Japanese Patent Laid-Open No. 61-156054.
Thus, the first object of the present invention is to provide a carbon black master batch for use in a toner capable of obtaining a toner which can make the carbon black be present stably in the conditions such that the carbon black has a small primary particle size and is dispersed homogeneously in the toner, can attain a high image density even with a small amount of the toner, and itself has a high blackness.
The second object of the present invention is to provide a toner for use in electrophotography (electrostatic charge image development) capable of yielding a desired image density by use of a smaller adhesion amount of toner (consumption amount), capable of preventing the occurrence of toner scattering and image fogging, and capable of thereby stably maintaining satisfactory image characteristics.
The third object of the present invention is to provide a toner for use in electrophotography (electrostatic charge image development) capable of yielding a desired image density by use of a smaller adhesion amount (consumption amount) of toner, capable of preventing the occurrence of toner scattering and image fogging, capable of thereby stably maintaining satisfactory image characteristics, and additionally capable of covering sheets of paper (media) with the toner particles leaving smaller blank space so that sheets of paper (media) become nondiaphanous, the concealment property is improved and the image density can be increased.